Indeed for everything there is a zeal, and for every zeal there is a slackening. So if its practitioner draws close and is moderate, then hope for his [success]. But if the fingers are pointed to him, then do not count him (among the worthy)

[Saheeh Targheeb (no. 57)]

Imam as-San’aani rahimahullah explained the Hadith saying:

‎”(الشره) is eagerness upon something. (For every zeal there is a slackening) that is: weakness and inactivity. A servant exaggerates in worship at first and with every exaggeration, there is a break and settling of the exaggeration. (الشره) means eagerness upon something. The Hadith is motivation for moderation in affairs and for the actor to continue upon it because “The most beloved actions to Allah, the Exalted are the ones that are consistent”. And also to avoid the side of extremism and negligence…(So if he is moderate) that is moderate and follows an upright path (Then hope for his success): that is: it is hoped he will be successful in his affair and complete what he entered into…”

Abdullah ibn ‘Amr (may Allah be pleased with him) said: It was mentioned to the Messenger of Allah that some men from his Companions were severely fatigued from worship. So the Messenger of Allah ‎(ﷺ) said: “That is the yearning of Islam and its slackening. For every yearning there is a slackening. So whoever’s slackening is towards the Book and the Sunnah, then he is upon the straight path. And whoever’s slackening is to disobedience, then that [one] is destroyed”

[Reported by Ahmad and graded Hasan by al-Albani in Saheeha (6/837-838)]

So we also learn the feeling of fatigue/tiredness after zeal and yearning was also felt by the Companions (may Allah be pleased with them). We should ensure that when we do slack that it is not in disobedience, but rather towards the Book and the Sunnah.

We should not give up, but instead work hard in light of the statement of the Prophet (ﷺ):

‎فَسَدِّدُوا وَقَارِبُوا وَأَبْشِرُوا

But try to be near to perfection and receive the good tidings that you will be rewarded